An organic light emitting display device using organic light emitting diodes (OLEDs) is provided with OLEDs and transistors (generally thin-film-transistors, TFT) for driving the OLEDs. Depending on the material used for the TFT's active layer, the TFT devices can be classified as a poly silicon TFT, an amorphous silicon TFT, or others. Depending on the geometrical structure of the TFT device, the TFT devices can be classified into a single gate (SG) and a double-gate (DG) structure, depending on the existence/nonexistence of a second gate of TFT. The DG TFT shown in FIG. 1 has advantages that its current responds more to the variation in gate voltage compared to the same gate biasing voltage of a SG TFT.
U.S. Pat. No. 7,414,600 discloses a circuit diagram of a unit pixel of a conventional active matrix OLED which employs an n-type DG TFT with the bottom gate grounded. The voltage programming type active matrix OLED includes two DG TFTs and one capacitor. A first bottom-grounded DG TFT serves as a switch. This switch, together with a capacitor, form a track-and-hold circuit for storing and maintaining the programmed pixel voltage. A second bottom-grounded DG TFT acts as a transconductance amplifier buffer which generates the output drain current for driving the OLED without loading the capacitor at its input gate. The second TFT is often referred to as “buffer” or “driver”. This type of circuit is sometime referred to as “voltage programming type” because input data are supplied in form of a voltage (Vdata). The current driving the OLED is then “derived” based on this input voltage (Vdata).
U.S. Pat. No. 7,532,187 discloses a similar example except that p-type TFT devices are deployed. Other than that the configuration and functional behavior of the circuit is similar to the circuit in U.S. Pat. No. 7,414,600.
SG TFT devices can be used in circuits shown in U.S. Pat. Nos. 7,532,187 and 7,414,600, and are also commonly deployed in active matrix displays.
The main drawback of TFT and similar devices is that their characteristics drift over time due to the continuous flow of current through them. There are multiple mechanisms responsible for this degradation including trapping of charge in broken bonds in the active material of the transistor. What complicates matters is that the degradation is not permanent and not equal over time or across different devices. The degradation primarily depends on the history of currents flowing through a particular TFT. Therefore, one method to correct for the degradation is to continuously “measure” and sample the amount of degradation and then corrected for by adjusting the input data.
It is clear that if the characteristics of the driving TFTs are not stable, the output current that drives OLED will not be stable. That is, the same input voltage will result in different OLED current, and therefore in different OLED brightness resulting in objectionable non-uniformity across the display. Sometimes, if a static image is driven and displayed for an extended period of time, followed by a second image, the imprint of the first image will be visible during the display of the second image, since the TFT devices drifted according to the intensity distribution of the first image. This is sometimes referred to as a “burn-in” effect; the first image is burned into the display. The pixel structures of U.S. Pat. Nos. 7,532,187 and 7,414,600 cannot correct the deterioration of the TFT driver threshold voltage and would demonstrate the degradation of the display.
Generally, the TFT degradation can be thought of as change or drift in the threshold voltage of the TFT. Several techniques have been proposed to address the drift and non-uniformity issues of TFT circuits (for example, Nathan, et al. U.S. Pat. No. 7,868,857). Unfortunately, many of these methods require additional TFT devices in the TFT circuits or require additional control lines to be supplied to pixels from the periphery of the circuit. Additional TFT devices make pixel electronics large, either reducing the fill factor of the pixel, or making the pixel large in size thus limiting the resolution of the display. Furthermore, many of these techniques require a modified technique for supplying Vdata, which makes the method very cumbersome and expensive to implement since it represents a significant departure from the current state-of-the-art in how the displays are driven.
An additional source of display degradation is aging of the OLEDs and drift in the OLED's characteristics. The degradation of the OLEDs could be referred to the input gate of the driving TFT and added to the degradation of the TFT threshold voltage. If one could somehow detect this combined degradation of the TFT and OLED one could adjust the input data voltage (VDATA) to compensate for the degradation at each pixel. Bu et al. U.S. Pat. No. 6,433,488 shows a technique that senses a current through an OLED, then programs the Vdata to achieve a target OLED current. Again, this technique is cumbersome, requires additional TFT devices in the pixel, and requires significant resources outside of the pixel array.
It would be desirable to have a method and a circuit that can compensate for drift in driving TFT characteristics, the drift in OLED characteristics, or both.